The present invention relates to the separation and purification of crude oil by means of a simplified apparatus and relates to an apparatus suitable for carrying out the above petroleum processing.
In the art of oil refining, it is common practice to perform an atmospheric distillation of crude oil having undergone pretreatments such as dehydration and desalting so that the crude oil is separated into bottoms and fractions of gas oil, kerosene, heavy naphtha, light naphtha, LP gas and light gas and to carry out hydrorefining for each of fractions to be subjected to hydrorefining, optionally followed by reforming. Thus, petroleum products are obtained. For example, referring to FIG. 5, the light gas (off gas) fraction among the fractions separated by an atmospheric distillation of crude oil has acid gases such as H2S separated by an amine treatment unit and is outputted as fuel gas. Among the above fractions, the LP gas fraction has its impurities removed by an LP gas treatment unit and is outputted as LP gas. Sulfur is recovered from the acid gases.
The light naphtha fraction undergoes treatment such as sweetening by a light naphtha treatment unit to thereby remove mercaptan, H2S, etc. and is formulated into gasoline. The heavy naphtha fraction undergoes a hydrorefining by a heavy naphtha treatment unit and, thereafter, a catalytic reforming and is outputted as gasoline. In the use of the heavy naphtha in the catalytic reforming, the sulfur content of the heavy naphtha must not exceed 1 ppm by weight. Thus, sulfur components such as mercaptan, undesulfurized sulfides and hydrogen sulfide (H2S) contained in the above hydrorefined heavy naphtha are removed by treating with an adsorbent of a metal oxide such as NiO, CuO or ZnO or by an amine absorption.
The kerosene and gas oil fractions are each individually hydrogenated, optionally followed by treatment with the use of, for example, an adsorbent, and outputted as kerosene and gas oil, respectively.
The atmospheric distillation bottoms are distilled at reduced pressure with the use of a vacuum distiller, and the thus obtained vacuum distillates are used as a feedstock for producing gas oil.
As is apparent from the above, in conventional oil refining, the individual fractions such as light naphtha, heavy naphtha, kerosene and gas oil are purified by the respective treatment units such as a hydrorefining unit. Therefore, problems are encountered such that the constitution of the petroleum processing apparatus is complex and such that not only are complicated and large facilities required but also construction cost is high.
Further, in conventional oil refining, it is conducted in unified form irrespective of the amount of processed crude oil. In this connection, it is desireable to simplify the petroleum processing apparatus and reduce the scale thereof to thereby lower oil refinery cost especially when the amount of processed crude oil is small.
In view of the above prior art, the applicant proposed a method comprising performing an atmospheric distillation of crude oil so that the crude oil is separated into bottoms and distillates and collectively hydrogenating the distillates in a reactor and an apparatus suitable for use in the method (see Japanese Patent Laid-open Publication No. 7(1995)-82573). In this method, the distillates are collectively hydrorefined and, thereafter, fractionated into individual fractions. This method enables simplifying the petroleum processing apparatus as compared with the prior art in which the respective hydrorefining reactors are employed for individual fractions. This method is useful especially when the amount of processed crude oil is small.
In the hydrodesulfurization of gas oil fraction containing sparingly desulfurizable sulfur compounds among the distillates obtained by the above atmospheric distillation of crude oil, the higher the desulfurization temperature, the higher the desulfurization efficiency. Thus, when the above distillates are mixed and collectively hydrogenated, it is necessary to select conditions under which gas oil can efficiently be desulfurized. However, when the hydrogenation temperature is 340xc2x0 C. or higher, sulfur components such as H2S having been removed by the hydrogenation reaction are likely to undergo a recombination reaction with olefin (naphtha fraction). When the catalyst life is close to an end (EOR: end of run), the hydrogenation reaction must be carried out at high temperature, thereby increasing the likelihood of recombination reaction.
When the sulfur content of light naphtha or heavy naphtha is increased by the above recombination, a new problem occurs such that the sulfur content of hydrogenated naphtha, especially, heavy naphtha may exceed the tolerance set for a feedstock for catalytic reforming.
When the collective hydrogenation of the distillates is performed at relatively low temperature for avoiding this problem, there occurs another problem such that the desulfurization efficiency is lowered with the result that only gas oil with a high sulfur content can be obtained.
The inventor has conducted investigations with a view toward solving the above problems once and for all. As a result, it has been found that the above object can be attained by performing the collective hydrogenation of distillates in two stages, i.e., the first stage comprising performing the hydrogenation at high temperature so that the desulfurization efficiency of gas oil is high and the second stage comprising performing the hydrogenation at low temperature so that the possibility of sulfur components such as hydrogen sulfide formed by the first-stage hydrogenation undergoing a recombination with olefin is very low. Further, it has been found that the above object can also be attained by separating hydrogenated oil which has been obtained by the first-stage hydrogenation and by subjecting only thus obtained heavy naphtha fraction to the second-stage hydrogenation, followed by an adsorption removal. The present invention has been completed on the basis of the above findings.
The diesel gas oil hydrogenating method in which the hydrogenation of gas oil is performed in two stages, i.e., the first stage comprising hydrogenating gas oil to thereby effect the desulfurization thereof and the second stage comprising hydrogenating the gas oil having been colored by the first-stage desulfurization so as to improve the hue thereof is known in the art.
For example, Japanese Patent Laid-open Publication No. 5(1993)-78670 describes the method in which diesel gas oil (petroleum distillate with a boiling point of 150 to 400xc2x0 C.) is hydrogenated at temperature as high as 375 to 450xc2x0 C. under a pressure of 45 to 100 kg/cm2 to thereby effect a desulfurization to a sulfur content of 0.05% by weight or below (first stage) and, thereafter, hydrogenating the gas oil at 200 to 300xc2x0 C. under a pressure of 45 to 100 kg/cm2 (second stage) so that the hue of the diesel gas oil having been colored by the first-stage hydrogenation is improved. Although the hue is improved to at least xe2x88x9210 in terms of Saybolt chronometry value in the second-stage hydrogenation, it is described in the Example portion of the literature that the sulfur content of the gas oil after the second-stage hydrogenation is the same as that of the gas oil after the first-stage hydrogenation, so that no desulfurizing effect is exerted in the second-stage hydrogenation. Furthermore, Japanese Patent Laid-open Publication No. 3(1991)-86793 proposed the similar method comprising desulfurizing gas oil (first stage) and performing a second-stage hydrogenation for improving the hue thereof (second stage). As in the above literature, it is described in the Example portion that no desulfurizing effect is exerted in the second-stage hydrogenation.
It is an object of the present invention to provide a petroleum processing method which enables the efficient separation and purification of crude oil by means of a simplified apparatus and to provide an apparatus suitable for carrying out the above petroleum processing.
The petroleum processing method of the present invention comprises the steps of:
performing an atmospheric distillation of crude oil so that the crude oil is separated into bottoms and distillates, these distillates comprising gas oil and fractions whose boiling point is lower than that of gas oil;
collectively hydrodesulfurizing the distillates in a reactor in the presence of a hydrogenation catalyst at 310 to 370xc2x0 C. under 30 to 70 kg/cm2G (first hydrogenation step); and
further collectively hydrodesulfurizing the above hydrodesulfurized distillates in a reactor in the presence of a hydrogenation catalyst at 280 to 330xc2x0 C. under 30 to 70 kg/cm2G (second hydrogenation step).
In this method, the second hydrogenation step is generally followed by the steps of:
separating gas fractions from the hydrodesulfurized distillates (gas separating step); and
separating the distillates having undergone the gas separating step into gas oil, kerosene, heavy naphtha and light naphtha fractions (fractionation step).
The heavy naphtha fraction obtained in the fractionation step can be catalytically reformed to thereby obtain gasoline. Generally, the heavy naphtha fraction has a sulfur content of not greater than 1 ppm by weight.
Alternatively, the petroleum processing method of the present invention may comprise the above crude oil atmospheric distillation step and first hydrogenation step followed by the steps of:
separating gas fractions from the distillates hydrodesulfurized in the first hydrogenation step (gas separating step);
separating the distillates having undergone the gas separating step into gas oil, kerosene, heavy naphtha and light naphtha fractions (fractionation step); hydrodesulfurizing the heavy naphtha fraction obtained in the fractionation step in the presence of a hydrogenation catalyst at 250 to 400xc2x0 C. under 3 to 30 kg/cm2G (second hydrogenation step); and
removing by adsorption sulfur components from the heavy naphtha fraction hydrodesulfurized by the second hydrogenation step (adsorption step).
In this method in which the fractionation step is carried out after the first hydrogenation step, the possibility of hydrogen sulfide undergoing a recombination reaction with olefin is very low in the second hydrogenation step conducted for the heavy naphtha. Thus, the second hydrogenation can be performed at higher temperatures than in the first hydrogenation step. The heavy naphtha fraction obtained in the adsorption step can be catalytically reformed to thereby obtain gasoline.
The petroleum processing apparatus of the present invention comprises:
an atmospheric distillation unit capable of performing an atmospheric distillation of crude oil so that the crude oil is separated into bottoms and distillates, said distillates comprising gas oil and fractions whose boiling point is lower than that of gas oil;
a first hydrogenation reactor capable of collectively hydrodesulfurizing the distillates separated by the atmospheric distillation unit; and
a second hydrogenation reactor capable of further collectively hydrodesulfurizing the distillates hydrodesulfurized by the first hydrogenation reactor.
This petroleum processing apparatus, generally further to the atmospheric distillation unit, the first hydrogenation reactor and the second hydrogenation reactor, comprises:
means for separating gas fractions from the distillates hydrodesulfurized by the second hydrogenation reactor; and
fractionating means for separating the distillates processed by the gas separating means into gas oil, kerosene, heavy naphtha and light naphtha fractions.
This petroleum processing apparatus may further comprise a catalytic reformer capable of catalytically reforming the heavy naphtha fraction separated by the fractionating means.
Alternatively, the petroleum processing apparatus of the present invention may comprise:
the above atmospheric distillation unit and first hydrogenation reactor;
means for separating gas fractions from the distillates hydrodesulfurized by the first hydrogenation reactor;
fractionating means for separating the distillates processed by the gas separating means into gas oil, kerosene, heavy naphtha and light naphtha fractions;
a second hydrogenation reactor capable of hydrodesulfurizing the heavy naphtha fraction separated by the fractionating means; and
an adsorber capable of removing by adsorption sulfur components from the heavy naphtha fraction hydrodesulfurized by the second hydrogenation reactor.
This petroleum processing apparatus may further comprise a catalytic reformer capable of catalytically reforming the heavy naphtha fraction processed by the adsorber.